The invention relates to oil refinery and petrochemical operations, and specifically to the treatment of off-gas streams containing hydrogen and mixed hydrocarbons from such operations by pressure swing adsorption and membrane gas separation.
Besides providing the octane level needed for gasoline products, the catalytic reformer is the principal hydrogen producer within a refinery. An important aspect of reformer operation is to generate as much hydrogen as possible, consistent with other requirements, of a quality suitable for use in the hydrogen-consuming units, particularly hydrocrackers and hydrotreaters.
The overhead vapor from the reformer reactors is typically split into at least two portions, one for recycle in the reactor loop, the other that forms a purge from the loop and that is the source of the net hydrogen product. This reactor purge stream is often sent to pressure swing adsorption (PSA) for upgrading to a high purity hydrogen product, typically containing 99% or more hydrogen. The hydrocarbon tail gas stream created when the PSA beds are regenerated is frequently treated as a waste gas stream and sent in its entirety to the plant fuel header.
Use of pressure swing adsorption (PSA) for this upgrading step is taught, for example, in U.S. Pat. No. 5,332,492, in which PSA tail gas is returned to the contactor section of the phase-separation steps, and U.S. Pat. No. 5,278,344, in which PSA is used to purify the net hydrogen stream after catalytic reforming and in front of a hydrodealkylation step.
It is also possible to use PSA to treat other diverse streams containing mixtures of hydrogen and hydrocarbons in refineries and elsewhere. Representative examples of such hydrogen-containing streams include overhead streams from fractionation columns used downstream of catalytic crackers, hydrocrackers and the like, overhead streams from cryogenic and other condensation units, overhead streams from absorbers, effluent streams from steam reformers, and refinery and petrochemical waste streams in general.
It is also known to use membrane separation for removing hydrogen from hydrocarbons in gas streams from various sources. U.S. Pat. Nos. 4,362,613, and 4,367,135, both to Monsanto, describe processes for treating the vapor from phase separators in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the reactor. A similar process is shown and described in U.S. Pat. No. 5,082,551, to Chevron. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L""Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from cracking effluent, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal. U.S. Pat. No. 5,785,739, to Membrane Technology and Research, describes a process for recovering light olefins from gas streams produced by a steam cracker, by a combination of condensation and membrane separation.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: xe2x80x9cPrism(trademark) Separators Optimize Hydrocracker Hydrogenxe2x80x9d, by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; and xe2x80x9cOptimizing Hydrocracker Hydrogenxe2x80x9d by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. The use of membranes in refinery separations is also mentioned in xe2x80x9cHydrogen Technologies to Meet Refiners"" Future Needsxe2x80x9d, by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons. Papers that specifically concern treatment of reformer off-gases are xe2x80x9cHydrogen Purification with Cellulose Acetate Membranesxe2x80x9d, by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; and xe2x80x9cPlant Uses Membrane Separationxe2x80x9d, by H. Yamashiro et al., in Hydrocarbon Processing, February 1985. In these papers, a system and process using membranes to treat the overhead gas stream from the absorber/recontactor section of the plant are described. All of these papers describe system designs using cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons.
A chapter in xe2x80x9cPolymeric Gas Separation Membranesxe2x80x9d, D. R. Paul et al. (Eds.) entitled xe2x80x9cCommercial and Practical Aspects of Gas Separation Membranesxe2x80x9d, by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063 and 4,836,833, to Air Products and Chemicals, and U.S. Pat. No. 4,892,564, to Cooley. U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
The use of rubbery polymeric membranes operated at low temperature to separate methane from nitrogen is taught in U.S. Pat. No. 5,669,958.
It has also been recognized that condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843, 5,374,300 and 5,980,609. Such combination is also described in patent application Ser. No. 09/316,507, now U.S. Pat. No. 6,159,272.
Numerous patents describe combinations of membrane separation with PSA. Representative examples include U.S. Pat. Nos. 4,229,188; 4,238,204; 4,398,926; 4,690,695; 4,701,187; and 4,783,203. U.S. Pat. No. 5,332,424, to Air Products and Chemicals, describes fractionation of a gas stream containing hydrocarbons and hydrogen using an xe2x80x9cadsorbent membranexe2x80x9d. The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Optionally, the membrane separation step is followed by PSA treatment. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,447,559; and 5,507,856, which all show combinations of carbon adsorbent membranes followed by PSA. U.S. Pat. No. 5,634,354 discloses removal of hydrogen from hydrogen/olefin streams. In this case, the membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen.
U.S. Pat. No. 5,435,836, concerns treatment of mixtures of hydrogen, carbon dioxide, carbon monoxide and methane from steam reformers. The gas mixture from the steam reformer is treated by PSA to recover a high purity hydrogen stream. The waste gas from the PSA unit is then treated by membrane separation using a carbon adsorbent membrane. The hydrogen-rich residue is returned to the PSA unit and the permeate gas from the membrane unit can optionally be used as fuel for the steam reformer. U.S. Pat. No. 5,753,010 discloses a process similar to that of U.S. Pat. No. 5,435,836, but in which the tail gas from the PSA unit is split into two fractions of unlike composition, which are treated separately in two discrete membrane steps.
U.S. Pat. No. 6,011,192 describes the use of a polymeric membrane unit before a PSA unit to remove C5-C8 hydrocarbons from the gas stream prior to its entry into the PSA separation step.
Patent application Ser. No. 09/083,784, now U.S. Pat. No. 6,190,536, describes treatment of off-gases from fluid catalytic cracking absorbers using hydrocarbon-selective membranes.
Patent applications Ser. Nos. 09/083,660 and 09/317,106, now U.S. Pat. Nos. 6,171,472 and 6,264,828, describe treatment of overhead gases in hydrocarbon conversion reactors of any type by passing gases in the reactor recycle loop across hydrocarbon-selective membranes.
Patent application Ser. No. 09/083,872, now U.S. Pat. No. 6,190,540, describes such a process applied specifically to hydrotreaters and hydrocrackers.
Patent application Ser. No. 09/316,508, now U.S. Pat. No. 6,179,996, describes such a process applied specifically to hydrogenation reactors.
Patent application Ser. No. 09/083,653, now U.S. Pat. No. 6,165,350, describes the use of hydrogen-rejecting membranes to directly treat overhead gases from the phase separators of catalytic reformers.
Patent application Ser. No. 09/471,302 copending and co-owned with the present application, describes the use of a membrane unit before a steam reformer to create a hydrogen-rich stream that bypasses the steam reformer and passes instead to PSA treatment.
Patent application Ser. No. 09/273,207 now U.S. Pat. No. 6,350,371, describes an improved catalytic reforming process in which PSA is used to recover hydrogen from the reformer off-gas and membrane separation is used to treat the PSA tail gas.
Patent application Ser. No. 09/316,498, now U.S. Pat. No. 6,183,628, of which the present application is a continuation-in-part, describes a process involving PSA, compression/condensation and membrane separation for separating hydrogen from hydrocarbons.
All of the copending and co-owned patent applications referred to above are incorporated herein by reference.
The invention is an improved process for treating mixed hydrogen/hydrocarbon effluent streams from refinery, petrochemical and other operations. The invention separates a stream containing at least hydrogen, methane, ethane and C3+ hydrocarbons into two or three streams: a hydrogen-enriched stream suitable for use as a source of hydrogen, an ethane-enriched stream suitable for use as fuel gas, and, optionally, a liquid C3+ hydrocarbon stream suitable for use as a source of LPG. Streams suitable for treatment by the process of the invention range from hydrocarbon lean streams, for example, containing less than 10% total hydrocarbons, to hydrocarbon rich streams, for example, containing as much as 50% or more total hydrocarbons. Typically the hydrocarbons present in the stream are in the C1-C8 range, with the majority of the content being C1-C6 hydrocarbons.
In its most simple form, the invention includes three unit operations or steps: a pressure swing adsorption (PSA) step to separate a high-purity hydrogen stream from the effluent stream; a compression/cooling step that may result in the formation of liquefied C3+ hydrocarbons; and a membrane separation step to separate remaining light hydrocarbons from hydrogen.
In a basic embodiment, these steps take the following form:
(a) passing a stream comprising hydrogen, methane, ethane and C3+ hydrocarbons through a pressure swing adsorption unit, thereby producing a hydrogen-enriched product stream and a tail gas stream;
(b) compressing and then cooling the tail gas stream, thereby optionally producing a condensed C3+ hydrocarbon stream and producing an uncondensed stream;
(c) passing the uncondensed stream across the feed side of a membrane separation unit containing a rubbery polymeric membrane having a feed side and a permeate side, and being selectively permeable to C1-C6 hydrocarbons over hydrogen;
(d) withdrawing from the permeate side a permeate stream enriched in ethane and C3+ hydrocarbons, and optionally enriched in methane, compared with the uncondensed stream;
(e) withdrawing from the feed side a residue stream enriched in hydrogen compared with the uncondensed stream.
Preferably, the membrane separation step of the process is carried out under conditions that provide a selectivity for ethane over hydrogen of at least about 3.5, more preferably at least about 4 and most preferably at least about 5.
In the case that the feed stream to the membrane separation unit is comparatively lean in C3+ hydrocarbons and the membrane separation step is performed at sub-zero temperatures, for example, it is also possible and it is desirable to carry out the process to provide a permeate stream that is enriched in methane as well as ethane compared with the membrane feed stream.
The tail gas from regeneration of the PSA beds is typically at comparatively low pressure, for example about 50 psia. Production of a liquid C3+ hydrocarbons stream in step (b) is facilitated, therefore, by compressing the tail gas to a few hundred psi in addition to cooling it. Compressing the tail gas stream also provides an increased driving force for the membrane permeation step. In addition, the compression/cooling step produces a membrane feed stream at a lowered temperature, which increases the methane/hydrogen and ethane/hydrogen selectivity of the membrane.
Preferably, the invention includes an additional step of recirculating at least a portion of the hydrogen-enriched membrane residue stream to the pressure swing adsorption unit, thereby increasing the amount of high-purity hydrogen produced by the process.
Optionally, at least a portion of the membrane permeate stream may be recirculated to the compression/cooling step.
The invention has an important advantage over other polymeric membrane separation processes that have been used in the industry in the past: all hydrocarbons, including methane, can permeate the membrane preferentially if desired, leaving a residue stream on the feed side that is concentrated in the slower-permeating hydrogen. The use of hydrocarbon-selective, hydrogen-rejecting polymeric membranes means that the hydrogen-enriched stream is retained on the feed side of the membrane. In other words, the hydrogen-enriched stream withdrawn from the membrane separation unit remains at pressure, which is desirable for recycle to the adsorption unit, as well as for facilitating delivery to other destinations in embodiments in which the residue stream is not recycled in the process. In contrast, hydrogen-selective membranes deliver a hydrogen-enriched stream at the comparatively low pressure of the permeate side, and this stream must almost always be recompressed for further treatment or use.
The use of polymeric materials for the membranes renders the membranes easy and inexpensive to prepare, and to house in modules, by conventional industrial techniques, unlike other types of hydrogen-rejecting membranes, such as finely microporous inorganic membranes, including adsorbent carbon membranes, pyrolysed carbon membranes and ceramic membranes, which are difficult and costly to fabricate in industrially useful quantities.
The invention has a number of advantages, including but not limited to:
increased hydrogen production compared with prior art techniques
production of a discrete LPG stream
ability to debottleneck plants where fuel gas production is at maximum.
Optionally, the hydrogen-rich stream from the membrane separation operation may be sent to a second membrane separation step to provide additional separation between the remaining light hydrocarbons and hydrogen. In this case, it is preferred to recirculate the permeate from the first membrane separation step to the compression/cooling step, to recirculate the hydrogen-rich residue from the second membrane separation step to the PSA step and to withdraw the ethane-enriched permeate from the second membrane separation step for use as fuel gas or to send to any other desired destination.
The preferred membranes used in the present invention can permeate all of the hydrocarbons, hydrogen sulfide and water vapor preferentially over hydrogen, and are capable of withstanding exposure to these materials even in comparatively high concentrations. This contrasts with cellulose acetate and like membranes, which must be protected from exposure to heavy hydrocarbons and water. If liquid water or C3+ hydrocarbons condense on the surface of such membranes, which can happen if the temperature within the membrane modules is lower than the upstream temperature and/or as the removal of hydrogen through the membrane increases the concentration of other components on the feed side, the membranes can suffer catastrophic failure. On the other hand, the membranes used in the invention preferentially and rapidly pass these components, so they do not build up on the feed side. Thus, the membranes can handle a wide diversity of stream types. This is a differentiating and important advantage over processes that have previously been available.
Most significantly, the invention provides processes that can separate C1-C2 hydrocarbons and C3+ hydrocarbons from hydrogen with a practical, industrially useful selectivity, and retain the hydrogen at high pressure.
It is to be understood that the above summary and the following detailed description are intended to explain and illustrate the invention without restricting its scope.